Bring up the person you want to modify, check the menu or the three dots and find customize. You 'll find (at least on mine) Backgrounds, conversation bubble, tones, and reset to default. chose Backgrounds, and at the bottom you'll see a big +, a paint pallet, default, classic, modern, time/clock, and some pictures. If you don't want one that is showing, choose the +. Here you will get choices; take a picture, collage, image search, apps, cloud, and pictures, and downloads. If you choose image search, type in a description of what you want.... ocean, sky, boats, flowers, etc, and it'll take you to a bunch of images. if you choose one (and mine's not working right right now and I think my LG v20 will go back to Verizon tomorrow) it will give you the choice of attaching the link or attaching the picture. Use Attach and it will download to your phone. Choosing the picture should get it onto the screen, then you back out of backgrounds and it should stay.
And you can also attach a notification to the person so you'll know who's texting you by the notification tone.